This invention relates to a liquid crystal element, and more particularly to a driving method of a ferroelectric liquid crystal element which exhibits an electro-optical memory function.
Ferroelectric liquid crystals are a series of compounds typified by (s) 2-methylbutyl-p-[(p-decycloxybenzylidene)amino]cynnamate (which is generally referred to as "DOBAMBC") whose molecular design and synthesis are made by Meyer et al in 1975 (Meyer et al, "J. de Phys.", 36 (1975) L-69). They exhibit a ferroelectric property in a smectic C* phase, for example. The ferroelectric liquid crystal molecules 1 form a layer structure and a helical structure in the smectic C phase as shown in FIG. 5. Incidentally, reference numeral 2 represents spontaneous polarization. It will now be assumed that a vector parallel to the long molecular axis is n, a permanent dipole moment perpendicular to the former is Ps, the angle between a layer normal and n is .theta., and a coordinates system is taken so that the layer normal and a Z axis becomes parallel. Then n and Ps can be expressed as follows: EQU n=(sin .theta. sin (2.pi.Z/Lo), sin .theta. cos (2.pi.Z/Lo), cos .theta.) EQU Ps/.vertline.Ps.vertline.=(cos (2.pi.Z/Lo), sin (2.pi.Z/Lo), O)
Although the ferroelectricity has been confirmed for some of the smectic phases other than C* phase, the description will be hereby made on the C* phase by way of example.
When an electric field higher than a threshold voltage Ec is applied perpendicularly to the helical axis, the molecules move and the helical structure becomes unwound while keeping the layer structure, and the permanent dipole moment perpendicular to the long axis of each molecule becomes parallel to the field. At the same time, not only the liquid crystal molecules in one layer but also those in all layers are arranged parallel to one another. Two kinds of state where the molecules are inclined at angles .+-..theta. can be attained as shown in FIG. 2(c) by selecting the direction of the field, and a display element or an optical shutter element can be fabricatd by either utilizing the birefringence or adding a dichroic dye to the liquid crystal.
When the field is removed, the ferroelectric liquid crystal molecules generally return to the original helical structure as shown in FIG. 2(b) due to their elastic righting moment of orientation, but a bistable state in which the helix is kept unwound can be accomplished such as shown in FIGS. 2(a) and 2(c) even at the time of zero field by, for example, positively utilizing the interface effect between glass and the liquid crystal by, for example, sealing the liquid crystal in an extremely thin cell which is about 1 .mu.m thick, as proposed by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, "Appl. Phys. Lett.", 36 (1980), 899; Japanese Patent Unexamined Publication No. 56-107216 corresponding to U.S. Serial No. 110,451 filed on Jan. 8, 1980, U.S. Pat. No. 4,367,924, etc.), or in a certain kind of smectic phase other than the smectic C phase.
As described above, the ferroelectric liquid crystal has the bistable state so that an electro-optical memory function can be accomplished. Therefore, the future applications of the liquid crystal include large-scale displays having a large number of picture elements, high precision displays, optical shutters, polarizers, and so forth. Although the possible application of the ferroelectric liquid crystal to high information capacity displays and the like has been discussed in the past, it has not been clarified in practice how to drive the liquid crystal by applying what voltage.